This invention relates to gas turbine type power plants adapted for industrial use and particularly to a means for increasing the temperature of the products of combustion produced by a centrifugal fluidized bed gasifier burning coal.
As is well known in conventional fluidized bed combustors air is fed to flow vertically upward through a bed which consists of solid particles. The flow of air upwardly forms a fluidized bed and the range of velocities of the fed air causes minimum fluidization conditions up to the value at which particle elutriation occurs. One of the limitations to using fluidized beds when being considered for gas turbine power plants is the low relatively heat release rate.
It is recognized that fluidized bed heat release rates are governed by the Reynolds Number, velocities and residence time of the particles in the bed. Because the drag of the smaller coal particles is very high due to the large cross sectional area relative to their mass these particles elutriate before they are combusted which lowers combustion efficiency and poses a potential turbine erosion problem.
Also, of paramount importance is the size of the bed required. In order to achieve a heat rate obtained from combustion of coal in the range where it is efficient or practical to operate a gas turbine the size necessary for the fluidized bed is very large and costly. For example, a fluidized bed measuring say 70' high and 20' in diameter would be necessary to operate, say the FT-50 gas turbine engine manufactured by the Pratt & Whitney Aircraft division of United Technologies Corporation, the assignee. The large size of the pressure vessel also requires long, large connecting pipes which introduce further complications due to the size, thermal expansions and cost.
The problems noted above can be obviated by forming the combustor into a rotating fluidized bed and flowing the fluidizing air radially inwardly toward the axis of rotation so that the air passes through the centrifugally retained bed. By imparting a high acceleration to the coal particles when in the centrifugal field, the through flow velocity, and, therefore, the heat release rate can be substantially increased, elutriation can be minimized so that both small and large particles are consumed, making the direct burning of coal for gas turbine engine possible.
Further, by introducing additives, such as Dolomite directly into the fluidized stream of coal in a manner to cause a high centrifugal field coupled to the rotation of the fluidized bed, (mechanically or aerodynamically) the sulfur and ash can be removed during the combustion of the coal, resulting in a clean uncontaminated continuous flow of high temperature air. The temperature of the combustor exhaust air would be sufficiently high, say 1600.degree. F. to drive the turbines of a gas turbine engine to produce power.
It is contemplated that for gas turbine power plant, say of the FT-4 or FT-50, both manufactured by the Pratt & Whitney Aircraft division of United Technologies Corporation, and using, for instance one combustor for the FT-4, or the case of the FT-50 using two combustors each combustor would produce between 20 and 30 megawatts of electrical or mechanical power. The active toroidally shaped fluidized bed would measure 8 feet in diameter, 2 to 21/2 feet deep and 12 feet long. The case or drum retaining the bed would rotate around 200 rpm and combustion exhaust gas separator would rotate at a higher speed than the drum and would measure 2' in diameter.
To achieve a higher turbine inlet temperature, in accordance with this invention a gasifier and a secondary combustor are employed.
The gasifier is run fuel rich by limiting the compressor discharge air to the fluidized bed and the combustible gas produced by the gasifier is mixed with the excess compressor discharge air in a secondary combustor. We have found that by this system the turbine inlet temperature can be increased at least by 500.degree. F.